Multiplier-divider is widely used on modern electronic devices. It aims to generate an output signal proportional to two or more input signals. The output signal may be voltage or current. One of the common applications of the multiplier-divider is on a PFC circuit to generate a control signal through an input current, a feedback signal and an input voltage.
These days safety regulations and power saving requirements are increasingly strict. Hence power supply usually has to equip with a PFC circuit to reduce resonance wave and regulate current phase to be close the voltage phase to improve power utilization efficiency. The conventional passive PFC circuit can improve the efficiency only about 70%, while the active PFC circuit can improve the power utilization efficiency above 80%. Hence the active PFC circuit becomes a necessary element for all almost types of power supplies in the future. The active PFC circuit can be divided into a discontinuous current mode and a continuous current mode. The continuous current mode is more suitable for the power supply with power output greater than 300 W, thus is the main R & D focus in the industry. The PFC circuit adopted the continuous current mode generates a control signal through a multiplier-divider to set current ON so that the continuous current forms an average current close to the output voltage phase. Therefore the multiplier-divider is an important and necessary circuit in the continuous current mode.
U.S. Pat. No. 7,057,440 entitled “Multiplier-divider circuit for a PFC controller” has two multiplier-divider units coupled in series and a pulse generator to regulate operation of the multiplier-divider units. Each multiplier-divider unit includes a charge time control circuit, a linear charge circuit and a sample circuit. It receives input of a first multiplier signal, a second multiplier signal and a divisor signal. It also has a current source to provide a selected current as the basis of gain. Output can be calculated according the following equation:
  Vo  ∝            I      R        ⁡          (                                    I                          A              ⁢                                                          ⁢              C                                ×                      V            E                                    V                      A            ⁢                                                  ⁢            C                    2                    )      
In the cited reference mentioned above, the multiplier-divider unit generates a charge signal VCHG through the first multiplier signal VE and a saw-tooth signal VSAW. The saw-tooth signal VSAW is generated by the divisor signal VAC. Moreover, the saw-tooth signal VSAW is transformed by a current I1 generated by the divisor signal VAC. But the current source of the divisor signal VAC that generates the current I1 has resistance (marked by numeral 122 in the drawings of the cited reference). Since an error occurs between two the integrated circuits due to different manufacturing processes, a difference occurs between the resistor 122 of the current source and the external resistance that causes a manufacturing variation while the current I1 is generated and passes through the resistor 122. As the divisor signal VAC. is generated by the current I1, it also contains an error resulting from the manufacturing variation. The error is proportional to the divisor signal VAC. As a result, output generated by the cited reference has an error of 1/m2 due to variations of temperature and manufacturing process. Such an error causes the multiplier-divider used on the active PFC circuit in the continuous current mode to regulate the current phase cannot increase the efficiency to a designed value. There is still room for improvement.